Internal combustion engines of vehicles may have operating modes which may be selectable by the driver. Thus in some vehicles an economy mode, a normal mode and a sport mode may be provided, each mode having a different engine response to one or more commands of the driver. Typically the engine may respond differently in each mode to a given input of the accelerator pedal, being least responsive in economy mode, and most responsive in sport mode. In this way driveability of the vehicle can be improved by providing a range of accelerator pedal movements which are appropriate to, for example, the desired output torque characteristic of the engine. Such a system necessarily relies upon an electronic input from the vehicle driver, for example an accelerator potentiometer providing an input signal to an electronic control unit having a plurality of accelerator pedal position/output torque maps, as will be further described. The maps may also be referred to as or accelerator pedal progression maps or pedal progression maps.
Another kind of operating mode relates to the terrain which a vehicle is intended to cross. U.S. Pat. No. 7,349,776, the content of which is hereby incorporated by reference, describes a vehicle control system in which the driver can implement improved control over a broad range of driving conditions, and in particular over a number of different terrains which may be encountered when driving off-road. In response to a driver input command relating to the terrain, the vehicle control system is selected to operate in one of a number of different driving modes including one or more terrain response (TR) modes. For each TR mode, the various vehicle subsystems are operated in a manner appropriate to the corresponding terrain.
In one arrangement, a mode (for example a winter mode) is available in which the vehicle is configured to launch from standstill in a forward gear other than first gear such as a second gear to reduce risk of excessive wheel slip. Different modes may have different accelerator pedal maps (amount of engine torque developed for a given accelerator pedal position), torque delivery (accelerator pedal maps in combination with a rate at which engagement of transmission clutches is controlled to take place thereby determining how aggressively gear shifts take place), and transmission shift points as a function of coefficient of surface friction. For example, in one or more TR modes the transmission shift points may be arranged wherein gear shifts take place at lower speeds than they might otherwise take place at, and in a more gentle manner (e.g. at a slower rate).
Thus one or more accelerator pedal position/engine torque maps appropriate to the terrain may be selectable by the driver. For example when driving in rocky terrain, a high torque output may be indicated for a small accelerator movement, thus giving immediate urge to overcome a rock step. In contrast on sand, a lower torque output may be indicated for the same accelerator movement, so as to avoid spinning a vehicle wheel and digging a hole. To some extent the selected torque map may be a matter of judgement related to the available grip on the terrain.
FIG. 1 shows a known motor vehicle 101 having a powertrain 101P. The powertrain 101P includes an engine 121, a transmission 124, a power take-off unit (PTU) 137, a rear driveshaft or propshaft 131R and a front driveshaft or propshaft 131F. The rear driveshaft 131R is operable to drive a pair of rear wheels 113, 114 via a rear differential 135R whilst the front driveshaft 131F is operable to drive a pair of front wheels 111, 112 via a front differential 135F.
The vehicle 101 has an engine controller 121C arranged to receive an accelerator pedal position signal from an accelerator pedal 161 and a brakes controller 141C operable to receive a brake pedal position signal from a brake pedal 163.
In the configuration of FIG. 1 the transmission 124 is releasably connectable to the rear driveshaft 131R by means of the PTU 137, allowing selectable two wheel drive or four wheel drive operation.
The PTU 137 is also operable in a ‘high ratio’ or a ‘low ratio’ configuration, in which a gear ratio between an input shaft and an output shaft thereof is selected to be a high or low ratio.
The high ratio configuration is suitable for general on-road or ‘on-highway’ operations whilst the low ratio configuration is more suitable for negotiating certain off-road terrain conditions and other low speed applications such as towing.
The vehicle 101 has a central controller 101C, referred to as a vehicle control unit (VCU) 101C. The VCU 101C receives and outputs a plurality of signals to and from various sensors and subsystems provided on the vehicle 101.
The vehicle 101 has a transmission mode selector dial 124S operable to select a required operating mode of the transmission 124. The selector dial 124S provides a control signal to a transmission controller 124C which in turn controls the transmission 124 to operate according to the selected mode. Available modes include a park mode, a reverse mode and a drive mode.
The vehicle 101 also has a terrain response mode selector dial 128S. The terrain response mode selector dial 128S is operable by a driver to select a required terrain response mode of operation of the vehicle.
It is to be understood that if a user selects the drive mode of the transmission 124, the engine controller 121C employs a drive mode accelerator pedal progression map to determine the amount of drive torque T that the engine 121 should produce as a function of accelerator pedal position P. If the user selects a ‘dynamic’ TR mode, the engine controller 121C employs a sport mode accelerator (or throttle) pedal progression map instead of the drive mode accelerator pedal progression map. The accelerator pedal progression maps differ in that the sport mode map is arranged to provide a more aggressive response by the engine 121C to a given initial advance (such as depression) of the accelerator pedal 161.
Different accelerator pedal progression map are also employed for different respective user-selectable terrain response modes.
As noted above, in some arrangements the vehicle may be operable automatically to select an appropriate TR mode for the prevailing driving conditions.
FIG. 2 shows two different accelerator pedal progression maps in the form of a plot of engine torque output T as a function of accelerator pedal position P on a scale from 0 (foot off) to 100% of full scale depression of the accelerator pedal 161. Two extreme vehicle operating modes A and B are illustrated. Mode A is an initially cautious accelerator pedal progression map (torque map) and may correspond for example to a TR mode suitable for use when driving over sand. Mode B is an initially more aggressive torque map and may correspond to a TR mode suitable for use when driving over rock. A driver may select operation according to mode A or mode B my means of the TR mode selector dial 128S. In some vehicles different torque maps may be selected automatically by the VCU 101C.
The response of the vehicle engine 121 to driver demand may be filtered to improve vehicle performance and to assist the driver to deliver abrupt or smooth acceleration and deceleration depending on the prevailing conditions. Thus a propulsion request may give a more or less immediate rise in torque T from the vehicle engine 121 according to an operating characteristic which is a function of vehicle operating mode.
A negative propulsion request (for example foot-off) may likewise give a more or less immediate reduction in engine output torque T. Thus for example, an engine response will be more immediate on a rock surface than on a sand surface. In this specification the term ‘propulsion request’ indicates a change of accelerator position P and includes both the positive propulsion request (increasing engine speed/torque) and the negative propulsion request (reducing engine speed/torque). By way of example, when driving on rocky terrain, on release of the accelerator pedal the torque may be reduced relatively quickly (and as possible in some embodiments) so as to minimise the chance of cresting an obstacle too quickly. Conversely on sand, it may be preferable to reduce the torque more slowly when the accelerator pedal is released. This is because high engine drag upon release of the accelerator pedal may cause one or more wheels of the vehicle to sink into the sand, making subsequent vehicle launch more difficult.
A propulsion request may comprise a movement of the accelerator pedal 161 from a zero accelerator position (foot-off) or may comprise a further movement from a non-zero position. Typically the position of the accelerator pedal P is continually sampled at a suitable frequency, and any change of position P may indicate a new propulsion request (positive or negative) to which the vehicle engine 121 is commanded to respond according to the selected map of accelerator pedal position P as a function of engine torque T.
In the alternative, a pedal progression map may relate accelerator pedal position P to an analogue of engine output torque, such as one or more of drive torque at the vehicle wheels, power output, another measure of tractive effort, fuel flow, air flow or any measurable indicator that varies according to a accelerator pedal progression and engine output torque. The engine output torque may represent the output of other forms of motive power, such as an electric motor, or an analogue thereof, such as motor current.
Many suitable analogues are known, so that whilst engine output torque is a convenient direct indicator, the use of one of more analogues to define the changing pedal progression characteristic is not excluded.
Furthermore accelerator pedal position P is typically converted into an electrical signal for use in a processor which commands an engine output torque according to the selected pedal progression map. The relationship between pedal position P and the electrical signal is defined by a characteristic, which may be linear so that the electrical signal changes proportionately to the increasing advancement of an accelerator pedal 161. The electrical signal is typically a voltage and the form of the electrical signal as a function of pedal position P will be termed the pedal signal map in this specification.
In the example of FIG. 2, it can be seen that at 50% pedal advance P, the solid line characteristic (11) corresponding to mode B shows the engine 121 developing about 100% torque T, whereas the chain-dot line (13) corresponding to mode A shows only about 25% of maximum torque. In the latter case 100% torque is provided only at 100% advancement of the accelerator pedal 161. The dashed line (12) is an intermediate characteristic C. The characteristic of mode A is typical for a soft terrain such as sand, whereas the characteristic of mode B is typical of a hard terrain such as rock.
Techniques are known for blending from one accelerator pedal progression map to another over time, so that the vehicle driver is not presented with a step change in engine output torque T as the mode changes instantaneously. This avoids for example an immediate increase in engine torque T from 25% to 100% of maximum torque at the 50% accelerator position P when the VCU 101C switches from mode A to mode B, i.e. from characteristic (13) to characteristic (11). A blend may take up to 20 seconds to complete.
FIG. 3 illustrates a known filter whereby a driver propulsion request is modified according to terrain conditions, so that an increase in engine torque T from zero is more rapid or less rapid in reaching the target output torque of the engine 121, as indicated by the selected pedal progression map. It is to be understood that by the term filter is meant that the driver propulsion request input by means of movement of the accelerator pedal P is acted on by the VCU 101C according to a characteristic that transforms an input to an output.
The filter characteristic may simply be a time-shift characteristic that delays the moment in time at which the driver propulsion request is acted on by the VCU 101C in commanding a corresponding increase (or decrease) in engine torque T. Thus, in the case the driver ramps the accelerator pedal 161 from a position P1 to a position P2 at a rate P′, the torque commanded by the VCU 101C may increase from torque T1 corresponding to position P1 to torque T2 corresponding to position P2 at a rate corresponding to P′, but only after a certain prescribed time period has elapsed from receipt of the signal. The time period may be settable (calibrateable) such that a manufacturer or maintenance personnel may set the value of this time period. The time period may itself be dependent on one or more parameters such as engine speed, vehicle speed, engine torque T, selected vehicle operating mode or any other suitable parameter.
Other filter characteristics may be useful in addition or instead. In practice the response of the engine 121 to generate an output according to a characteristic of FIG. 2 is more delayed or less delayed by the VCU 101C, but to the intent that target torque T is reached within a maximum period, for example about two seconds.
In FIG. 3 accelerator pedal position P is plotted against time t for a given output torque characteristic. The solid line 21 represents pedal position P, and shows a rapid advancement from minimum to maximum (100%) within a short period of time.
The dotted line 22 represents engine output torque T for one operating mode of the vehicle, where the propulsion request (pedal position P, line 21) gives an immediate engine response. This engine output torque T (line 22) closely follows the changing accelerator pedal position P, and the speed of response of the engine 121 is substantially unmodified. This operating mode is typical for a hard terrain, so that engine output torque T reaches the target torque substantially immediately as indicated by the selected pedal progression map.
The dashed line 23 represents engine output torque T according to a more filtered propulsion request for an operating mode typical for a slippery surface such as grass, gravel or snow. The unfiltered propulsion request in this example corresponds to line 21 of FIG. 3. In this case the propulsion request causes engine output torque T to rise more slowly so as to avoid spinning of the vehicle wheels 111-114. In the case of line 23, the rate of change of torque T is somewhat reduced as compared with characteristic 22.
The chain-dot line 24 of FIG. 3 represents more extreme filtering appropriate for an operating mode typical of sand, where wheel spinning may cause the vehicle 101 to dig a hole and become stuck. In this case engine torque T rises still more slowly in response to a propulsion request.
The characteristics of FIG. 3 are merely examples, and can be modified by a suitably skilled person to obtain an engine response most suitable for the vehicle 101 and the operating condition thereof. In the illustration, target output torque T may be achieved within 1-2 seconds for all characteristics although other values are also useful.
It is to be understood that the same techniques can be applied to a negative propulsion request, so that engine output torque T will fall more quickly or less quickly according to the selected pedal progression map.
In practice, all propulsion requests may cause a fitter to be applied, and the filter may be negligible (e.g. characteristic 22) or significant (e.g. characteristic 24). In the vehicle 101 of FIG. 1, characteristic 22 corresponds to a rock crawl mode whilst characteristic 24 corresponds to a sand mode.
Each characteristic is typically retained in a memory device of an engine control unit (ECU) such as engine controller 121C or VCU 101C and is automatically implemented upon selection or detection of a new vehicle operating condition.
The example described above with respect to FIG. 3 applies a filter to a pedal progression map relating accelerator pedal position P to engine output torque T. In the alternative it is also possible to apply a filter to the pedal signal map to achieve a similar (or same) effect. This detection of a new vehicle operating condition will result in the selection of an appropriately filtered pedal signal map, to the intent that engine output torque T will rise or fall at a rate appropriate to the newly detected operating condition.
If an alternative operating mode is selected by the vehicle driver, a change in the characteristic of engine response to a propulsion request is generally not a surprise—the change is in fact expected by the driver and is generally desirable. However difficulties may arise if the operating mode is automatically selected in response to a vehicle sensing a change of operating condition. Thus, for example a vehicle may include a system to detect a change of terrain from rock to sand and command the vehicle engine to adopt a different torque map. The consequent change in engine response may be disconcerting to the driver, especially if such automatic mode changes are repeated frequently.
For example, if driving on a sand terrain, the vehicle driver may become accustomed to a somewhat delayed engine response. The vehicle may automatically detect a temporary change to a hard surface, such as rock, and consequently change the operating mode of the vehicle so that the engine response is immediate.
A mode change may cause an acceleration request to be implemented more or less quickly, as described above, and may also cause a more or less instant drop in engine torque T in response to reverse movement of the accelerator pedal. A sudden drop in torque T may be obviated for certain terrain conditions.
Thus with reference to FIG. 3 a switch from the sand mode (line 24) to the rock crawl mode (line 22) one second after an acceleration request results in an immediate doubling of engine output torque T. The opposite effect may occur when the accelerator pedal is released. In the case of a switch from rock crawl mode to sand mode one second after the acceleration request, an immediate halving of engine output torque T takes place.
This change may be disconcerting, and it would be desirable to indicate to the driver that such a change is in progress; visual or audible indicators are not considered desirable because the driver may be overloaded with information, and not immediately appreciate what the indicator is indicating. Furthermore automatic mode changes may occur frequently, and such indicators may be a distraction.
What is required is an improved means and method of allowing the vehicle driver to become accustomed to a change of engine torque characteristic in response to a propulsion request when a mode change takes place.
It is against this background that the present invention has been conceived. Aspects and embodiments of the invention may provide a method, a system or a vehicle which address or obviate the above problems. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.